Projection systems, for projecting an image on to a screen, use several different components for providing efficient illumination of an imager. Projection systems typically use a lamp generate the illumination light, with several optical elements being interposed between the lamp and the imager to transfer the light to the imager. An imager is an article that impresses an image on a beam of light. It may perform this function through different mechanisms, for example by absorption, as with a photographic slide, by polarization, as with a liquid crystal display (LCD), or by diverting the light, as with a micromechanical array of individually addressable, tiltable mirrors. Some imagers, such as a photographic slide, are made to impose color on the beam of light inherently. Others, such as the micromechanical array of tiltable mirrors, require that the color be imposed by breaking the light into primary colors and supplying them separately to the imager or imagers. If the number of imagers present in the projection system is less than the number of primary colors, then the different colors are normally supplied to the imager one after the other. This process is referred to as field sequential color. If the number of imagers is equal to the number of primary colors, then the light can be supplied to the imagers simultaneously. Both methods are widely used.
Different methods are used to homogenize the intensity of the light incident on the imager or imagers from the lamp. Tunnel integrators are one approach of homogenizing the light intensity. The tunnel integrator is typically a tube or rod, often circular in cross-section although this is not a requirement. The light, directed into one end of the tunnel integrator, passes along the tube or rod via multiple reflections at the wall of the integrator to the output end. The multiple reflections within the tube or rod result in the output from the tunnel integrator being more uniform than at the input end. The light may be reflected within the rod using total internal reflection, or using front surface reflection. A hollow tunnel integrator using front-surface reflection does not refract the incoming light, and so may be able to homogenize the incident light over a shorter distance than an internally reflecting tunnel integrator.